Osteopenia is a serious and growing public health concern for women. Untreated osteopenic women are at high risk of becoming osteoporotic, and also are at greater risk for fractures than women with normal bone mineral densities (BMD). Low BMD-related fractures are associated with significant long-term impairment, high morbidity rates and high medical costs. In contrast to the treatment guidelines for osteoporosis, optimal preventive and sustainable interventions for osteopenic women are not yet well-defined. Current guidelines for the treatment of osteopenic women generally include the recommendation of regular exercise. Tai Chi, a mind-body exercise that is growing in popularity in the U.S., shows strong potential as an effective, sustainable, safe and practical intervention for women with low bone density. Preliminary studies suggest Tai Chi can reduce rates of BMD decline in post-menopausal women. While suggestive, these studies are small, have numerous design limitations, and include primarily Asian women who are known to have lower BMD and different bone geometry and fracture risk than other ethnic groups. Tai Chi has also been shown to be effective in improving balance, reducing falls and increasing musculoskeletal strength and flexibility. Moreover, Tai Chi has been shown to be safe in aging and de-conditioned populations. We propose a pilot randomized controlled trial to assess the efficacy and feasibility of Tai Chi as an adjunct to standard care for post-menopausal osteopenic women. Forty-eight osteopenic (T-scores -1.0 to -2.5) women ages 45-65 will be recruited from a large multispecialty group practice. Our primary aim is to assess the feasibility of recruiting and retaining osteopenic women into a randomized controlled trial of 9 months of Tai Chi. Our secondary aim is to collect preliminary data on the efficacy of Tai Chi in reducing rates of bone loss in osteopenic women using sensitive markers of bone turnover and dual-energy x-ray absorptiometry. For a subset of participants (n=16), biomechanical responses to Tai Chi (e.g. bone-specific mechanical loading and postural stability) will also be assessed. The results of this study will inform the design of a future trial evaluating the benefits and safety of Tai Chi for osteopenic women, as well as the biomechanical and physiological mechanisms through which Tai Chi may impact BMD and fracture risks associated with osteopenia.